Physical
Properties of Gemstones

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The Effects of Heat

When placed before the flame of the blowpipe, some minerals fuse
entirely, whereas only the surface consistency of others is affected.
The results are used in the identification of rough minerals. Such methods
are useless in identifying fashioned gemstones, since their beauty and
value would be partly or wholly destroyed. However, it is very important
for the the repairman to be familiar with the effects of heat on various
gemstones.

Of the important gem minerals, turquoise, emerald, garnet, quartz,
peridot, zircon and tourmaline are likely to crack under heat unless
well protested from direct blowpipe flame and cooled slowly. Turquoise
is so vulnerable to direct heat that even the low temperature produced
by a burning match may cause it to fly into may pieces. Fortunately,
other gemstones are not as easily damaged by heat. In addition, a number
of stones are likely to change color when heat is applied. These include
amethyst, aquamarine, zircon, tourmaline and topaz. If subjected to
sufficiently high temperatures (about 900°C)
for a long enough period, diamond will first cloud and then blacken
on the surface, and may even be totally destroyed by burning up.

From the number of stones affected, it can be seen that it is important
for the repairman to be careful when sizing rings and doing other types
of repair work involving the use of heat.

The Effects of Chemicals

Certain elements or combinations of elements, especially acids,
have a visible effect on the surface of certain minerals, and more especially
on the powder of a mineral. The effect of such chemicals is used to
great advantage in identifying rough minerals, but would affect the
beauty and value of a fashioned gem.

Hydrofluoric acid is sometimes recommended as a test. It dissolves
glass but etches the surface of the quartz gems; therefore, it is of
value number of gemstones are affected by acids, most transparent stones
are affected very little. Many of the translucent and opaque stones
are affected materially. Both natural and cultured pearls are dissolved
by hydrochloric and some other acids. All acids affect pearl, coral,
malachite, azurite and turquoise. The effect of hydrochloric acid is
sometimes used to separate imitation pearls from cultured and natural
pearls. A drop of the acid on any pearl will cause strong effervescence
but will not affect the imitation. Since the acid will damage a pearl
unless applied in a minute droplet and removed quickly, extreme care
must be exercised in using the test.

Feel

Some minerals may be recognized by their feel, but the highly polished
surfaces of gems so greatly lessens the possibility of using feel for
identification that it is only mentioned in connection with a few stones:
Jade (greasy), topaz (usually slippery), pearl (gritty to the teeth),
some chalcedony (adheres to the tongue). Those who experiment over a
number of years may develop a valuable sense of feel.

Odor

Some minerals have characteristic odors reminiscent of garlic, horse
radish, sulphur, resin, etc, when heated, rubbed, breathed upon or struck.
Rarely, however, does odor prove valuable in gem testing. Almost the
only use is in distinguishing between amber and its substitutes.

Magnetism

A very few minerals in their natural state are capable of being
attracted by a strong steel magnet; they are often said to be magnetic.
These minerals are seldom of gemological importance. Hematite is sometimes
slightly magnetic, but this is probably due to a small admixture of
magnetite, which has the property of being attracted by a magnet. Magnetite,
pyrrhotite and some varieties of native platinum are magnetic. The variety
of magnetite known as LODESTONE exhibits the attracting
power and polarity of a true magnet. These are sometimes called "natural
magnets" and are occasionally used for pocket pieces.

Electricity

An electrical charge can be developed in many substances and in some
degree, in all minerals by either friction or heat. Electricity developed
by heat is Called PYROELECTRICITY. Tourmaline is pyroelectric,
and topaz and amber are examples of materials that become high electrified
by friction.

Fluorescence and Phosphorescence

Some minerals, when exposed to invisible ultraviolet or cathode rays,
will emit visible light. This property is known as FLUORESCENCE.

It is named from the mineral fluorite, which often displays the phenomenon
to a market degree. Almost every genuine or synthetic ruby fluoresces
a bright red when exposed to ultraviolet light. When the glow continues,
or "coats" after the exciting rays have been cut off, the phenomenon
is known as PHOSPHORESCENCE.

Many diamonds fluoresce in ultraviolet light and some will phosphoresce.
The significance of fluorescence in diamonds lies in the fact that most
of the stones that do glow under ultraviolet light exhibit a blue color,
The presence of ultraviolet in day light causes diamonds that fluoresce
a strong blue to take on a blue cast that is not evident in artificial
light (which lacks ultraviolet). Thus diamonds that fluoresce blue have
a much better color in daylight than in artificial light. Some jewelers
have used this property to advantage in creating interest in diamonds.
Although it has little effect on value, it does make some customers
feel that their diamond is unusual and unlike those of their friends.

Since various gemstones react differently under ultraviolet light,
characteristic fluorescence can sometimes be used as a means of identification.
Usually, however it is used more as an indication of identity rather
than as a means of conclusive identification.